The Ministry of Defence is refusing to release any information about the replacement of the Trident nuclear missile system, including the costs and even whether it is needed to deter an enemy.

Its blanket dismissal of requests under the Freedom of Information Act was posted yesterday on the MoD's website. Officials said refusal was on the grounds of national security and the public interest, though they admitted that Trident replacement is a "topical issue at present".

The MoD said its decision was taken with the approval of the defence secretary, John Reid. The department was asked to release assessments it has made of the threats that might be deterred by any Trident replacement. Though there was a "strong public interest" in having a "credible nuclear deterrent", it was not in the public interest to publish its assessments about what threats a Trident replacement could deter.

The MoD refused to disclose the nature of discussions with America on Trident on the grounds that "there is a public interest in the UK maintaining strong relations with the US". That would be prejudiced if any information about talks was released.

It "neither confirms nor denies" whether it holds information on the costs of replacing Trident. The government says a decision whether to replace Trident will have to be taken during this parliament.